The present invention relates to target seeking systems for receiving radiation of two different frequencies simultaneously, for example electro-optical and microwave or radio frequencies (RF), and is particularly concerned with dual mode target seeking systems for airborne vehicles such as guided missiles, for example.
Missile target seeking systems receive target generated or target reflected radiation from moving or stationary targets and determine the relative positions of the missile and target from the information received. This information is used to make adjustments to the missile flight path so that it is accurately directed at the target. In the past missile target seeking systems have included both electro-optical radiation detection systems and radio or microwave frequency tracking systems. The electro-optical system is passive and detects radiation generated by the target, for example infrared radiation. The radio frequency system transmits radiation to the target and receives radiation reflected from the target. Since both systems have disadvantages when used alone, it is normally desirable to combine both electro-optical and radio frequency tracking in a missile target seeking system to ensure relatively high accuracy.
Because of the lack of space and gimbal mounting requirements, and weight restrictions in a missile seeker head, both the electro-optical and radio frequency target seeking system are normally commonly mounted to utilize a common aperture. In previous integrated systems of this type conventional flat plate arrays or parabolic reflectors for the radio frequency portion of the system are combined with the infrared system. This integration results in interference between the two scanning systems and significant aperture blockages. This leads to degradation in the radio frequency signal pattern, for example, higher sidelobes, and also to lower radio frequency gain.
Some previous combined electro-optical and radio frequency target seeking systems of the parabolic reflector type are shown, for example, in U.S. Pat. No. 2,972,743 of Svensson et al and U.S. Pat. No. 3,114,149 of Jessen. In both of these systems incoming infra red radiation is reflected from two infra red reflective surfaces into an infrared detection system. Microwave radiation is reflected from a parabolic reflector onto a microwave receiver/transmitter centrally mounted in one of the infrared reflective surfaces. The infra red detector system projects through the other reflective surface. These systems have mounting and space problems, particularly when used in missile seeker heads, and give rise to significant interference and aperture blockages since the microwave receiver and infrared reflector are mounted in the path of incoming rays.
A flat plate array antenna system is shown in U.S. Pat. No. 3,701,158 of Johnson. A radio antenna comprising a plate array of dipole radiating slots for receiving and transmitting directional radiation has an infra red sensing system extending perpendicularly out of it. The central cut out in the antenna array for mounting the infrared system will result in some loss in the radio frequency signal pattern.